1. Field of the Invention
The present invention generally relates to loudspeakers, and more particularly to a moving-coil planar speaker.
2. Description of Related Art
As shown in FIGS. 1 and 2, a conventional moving-coil planar speaker usually contains a frame 1, a conepaper or diaphragm 2, a driver 3, and a surround 4. The frame 1 is basically has flat rectangular shape with a front opening. The frame 1 is mainly formed by a rectangular side wall 102 perpendicularly surrounding the circumference of a back wall 108. The side wall 102 is composed of a pair of opposing short wall segments 104 and a pair of opposing long wall segments 106. Assuming that the short wall segments 104 have a length a and the long wall segments 106 have a length b, then a is less or equal to b. The driver 3 is fixedly positioned at the center of the back wall 108 and is enclosed by the side wall 102. The surround 4 is a ring of flexible material (such as foam or rubber) that suspends the diaphragm 2 in the front opening of the frame 1, thereby sealing the driver 3 inside. The cross section of the surround 4 has an arc shape with an inner edge and an outer edge fixedly joined to the circumferences of the diaphragm 2 and the side wall 102, respectively. The diaphragm 2 therefore can freely vibrate in the front opening of the frame 1.
The driver 3 mainly contains a magnet set 302, a voice coil 304, an inner frame 306, and a spider 308. The magnet set 302 is fixedly housed inside the inner frame 306. The voice coil 304 has a front end fixedly attached to a back surface of the diaphragm 2 and is supported in a front opening of the inner frame 306 by the spider 308 which is a ring of flexible material. The magnet set 302 is threaded in a back end of the voice coil 304. The spider 308 holds the voice coil 304 in position, but allows it to move freely back and forth along its axis in the magnet field produced by the magnet set 302.
When electrical current is introduced through the voice coil 304, a electromagnetic field is produced to interact with the magnetic field produced by the magnet set 302. The interaction between the two magnetic fields causes the voice coil 304 to move back and forth along its axis. When the voice coil 304 moves, it pushes and pulls the diaphragm 2. The diaphragm 2 therefore vibrates the air in the front, thereby creating sound waves. As such, depending on how fast and how strong the diaphragm 2 vibrates, sounds of various frequencies and amplitudes are produced.
Generally, a speaker's performance in the low frequency range is measured by its dynamic range and bandwidth. Dynamic range is about the sound pressure produced by the speaker. If the diaphragm can drive more amount of air, the speaker then can produce a stronger sound pressure, thereby achieving a superior dynamic range and efficiency. Therefore, the dynamic range of a speaker is positively related to the piston area and displacement of the speaker's diaphragm. Among them, the displacement of the diaphragm (or the amplitude of the diaphragm's vibration) is determined by the magnetic force between the voice coil and the magnet set. Generally, greater dimensioned magnet set and voice coil imply that the driver is capable of driving the diaphragm to vibrate with greater amplitude and thereby producing a louder sound and a superior dynamic range.
On the other hand, the low-frequency bandwidth is directly affected by the sizes and flexibilities of the surround and spider. A spider with greater outer diameter is more flexible and the speaker would therefore have a broader low-frequency range. In contrast, a spider with a smaller outer diameter is more rigid. The speaker's frequency response is shifted towards the mid- and high-frequency ranges and the speaker thereby suffers a less satisfactory low-frequency performance.
When the short wall segments of a conventional planar speaker is reduced down to a<b/2, the low-frequency response of the speaker would deteriorate significantly. On one hand, if the spider's outer diameter is reduced to fit in the narrow frame, the flexibility of the spider would decrease and the low-frequency bandwidth of the speaker would be reduced as well. On the other hand, if the driver's outer diameter and the size of the magnet are reduced to maintain the original bandwidth, the driver wouldn't be able to exert enough driving force and the speaker's dynamic range would be seriously affected. In other words, conventional speaker design approaches are not appropriate for the elongated planar speaker. Moreover, when the diaphragm is driven to perform a back-and-forth movement under a middle frequency, the diaphragm would exhibit a symmetric deformation along the speaker's longer side. One such symmetric deformation, referred to as the first symmetric bending mode is shown in FIG. 3 in which the side view of the diaphragm shows that the area of the diaphragm contains two out-of-phase zones. There are other possible symmetric deformations where the area of the diaphragm is separated into more out-of-phase zones. As illustrated, the dashed line connecting the points i and j is referred to as the cross-over line and the points i and j are referred to as cross-over points. The cross-over line separates the area of the diaphragm into two zones, marked as A (center zone) and B (end zone) in FIG. 3. The two zones of the diaphragm, one in front of and the other one behind the cross-over line have a 180-degree phase difference. That is, when zone A moves in one direction, zone B moves in the other direction. The sound pressures produced by the two zones would interfere with and cancel each other. FIG. 4 illustrates the result of such cancellation. In FIG. 4, the vertical axis is the sound pressure level (SPL) and the horizontal axis is frequency. As illustrated, the sound pressure level has a sharp drop around 2 KHz. Such a sudden sound pressure drop in the middle-frequency range would seriously affect the reproduced sound quality of the speaker.